Throughout the 20th century, millions of Greeks migrated to the United States, United Kingdom, Australia, Canada, and Germany, creating a large Greek diaspora. Net migration started to show positive numbers from the 1970s, but until the beginning of the 1990s, the main influx was that of returning Greek migrants or of Pontic Greeks and others from Russia, Georgia, Turkey the Czech Republic, and elsewhere in the former Soviet Bloc.
A study from the Mediterranean Migration Observatory maintains that the 2001 census recorded 762,191 persons residing in Greece without Greek citizenship, constituting around 7% of total population. Of the non-citizen residents, 48,560 were EU or European Free Trade Association nationals and 17,426 were Cypriots with privileged status. The majority come from Eastern European countries: Albania (56%), Bulgaria (5%) and Romania (3%), while migrants from the former Soviet Union (Georgia, Russia, Ukraine, Moldova, etc.) comprise 10% of the total. Some of the immigrants from Albania are from the Greek minority in Albania centred on the region of Northern Epirus. In addition the total Albanian national population which includes temporary migrants and undocumented persons is around 600,000.
The 2011 census recorded 9,903,268 Greek citizens (91,56%), 480,824 Albanian citizens (4,44%), 75,915 Bulgarian citizens (0,7%), 46,523 Romanian citizenship (0,43%), 34,177 Pakistani citizens (0,32%), 27,400 Georgian citizens (0,25%) and 247,090 people had other or unidentified citizenship (2,3%). 189,000 people of the total population of Albanian citizens were reported in 2008 as ethnic Greeks from Southern Albania, in the historical region of Northern Epirus.
The greatest cluster of non-EU immigrant population are the larger urban centers, especially the Municipality of Athens, with 132,000 immigrants comprising 17% of the local population, and then Thessaloniki, with 27,000 immigrants reaching 7% of the local population. There is also a considerable number of co-ethnics that came from the Greek communities of Albania and the former Soviet Union.
Greece, together with Italy and Spain, is a major entry point for illegal immigrants trying to enter the EU. Illegal immigrants entering Greece mostly do so from the border with Turkey at the Evros River and the islands of the eastern Aegean across from Turkey (mainly Lesbos, Chios, Kos, and Samos). In 2012, the majority of illegal immigrants entering Greece came from Afghanistan, followed by Pakistanis and Bangladeshis. In 2015, arrivals of refugees by sea had increased dramatically mainly due to the ongoing Syrian civil war. There were 856,723 arrivals by sea in Greece, an almost fivefold increase to the same period of 2014, of which the Syrians represent almost 45%. The majority of refugees and migrants use Greece as a transit country, while their intended destinations are northern European Nations such as Austria, Germany and Sweden.